BRIDGEVIEW, Ill. – The Chicago Fire could be forced to keep their playoff hopes alive against Seattle on Saturday without Designated Player Nery Castillo, thanks to a hamstring injury suffered last weekend against Real Salt Lake.

Castillo – who was not listed on the league's injury report released Tuesday – did not train on Wednesday or Thursday after suffering the injury in the Fire's 1-0 loss to RSL last Saturday at Rio Tinto Stadium.

Fire head coach Carlos de los Cobos told reporters on Wednesday that he’ll use a 4-5-1 formation if Castillo is unable to play. Collins John would almost certainly start as the lone forward and Freddie Ljungberg, Patrick Nyarko, Marco Pappa, Logan Pause and Bratislav Ristic would likely get the nods in the midfield should Castillo be unavailable.

The Fire used a 4-5-1 in the beginning of the season before switching to a 4-4-2 earlier this summer.

“When we started this season we played with only one forward and we had good moments,” de los Cobos said. “In the last matches I played with two forwards. ... I need to decide if I play with two forwards or only one forward depending on what happens with Nery.”

If Castillo is healthy, expect him to start with John up top in a 4-4-2, pushing Ristic to the bench.

Sam Stejskal covers the Chicago Fire for MLSsoccer.com. Email him at sam.h.stejskal@gmail.com and follow him on Twitter @samstejskal.